1. Field of the Invention
This invention relates to a method of and an apparatus for molding an article having undercut portions such as cross holes provided in the inner peripheral surface of a helicoid screw or a cylinder member by the use of a core.
2. Description of the Prior Art
Various methods of molding a member having undercut portions have heretofore been proposed. As one of them, there is the open core mounted on a mold for injection molding shown in U.S. Pat. No. 4,130,264. However, the open core disclosed in this patent adopts a construction as shown in FIG. 1 of the accompanying drawings wherein slide cores 2 and 4 for forming an undercut portion relative to a core bar 1 are moved as indicated by arrows A, B and C in the thrust direction relative to an axis O.sub.1 --O.sub.2, and in any of a case where the slide cores are opened and a case where the slide cores are closed, the core bar 1 and the slide cores 2 and 4 are positioned by a guide member not shown in FIG. 1. Therefore, high accuracy of the molded article cannot be ensured unless the accuracy of the angle of inclination .alpha. of the slide core 2 with respect to the core bar 1 and the accuracy of the angle of inclination .beta. of the slide core 4 with respect to the core bar 1 and the accuracy of fitting of the slide cores 2 and 4 are kept high. Thus, the finish accuracy and durability of the metal mold are greatly required, and this involves much difficulty in the manufacture of the metal mold. Also, to ensure smooth relative movement between the core bar 1 and the slide cores 2, 4, a clearance is required therebetween, but such clearance greatly affects the finish accuracy of the molded article and it becomes difficult to obtain products of high accuracy.